Apparatus for dynamic termination logic signaling

ABSTRACT

An information handling system includes a plurality of transmission lines coupled together at one end and having a characteristic impedance, a driver circuit coupled to one of the transmission lines, a plurality of receiver circuits individually coupled to distinct ones of the transmission lines for resolving the signals, and on-chip terminators having a output impedance corresponding to the characteristic impedance and that can be coupled or decoupled from a node by on-chip circuitry. In this embodiment, the terminators are separate and distinct from driver circuitry. However, when a node is in a receive configuration, its corresponding termination resistor is tied to the transmission line at that node, and its corresponding driver circuit presents a high impedance output to the node. A second embodiment provides driver circuits having particular on-chip pull-up and pull-down resistances, one of the either the pull-up or pull-down resistances being used to terminate the bus for those nodes which are in a receive configuration. In this second embodiment, separate terminators are not required, due to the driver operating as the terminator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application relates to co-pending U.S. patent application Ser. No.08/881,939 attorney docket number SP-2086 US, filed on Jun. 25, 1997,entitled Impedance Control Circuit and naming Sai V. Vishwanthaiah,Jonathan E. Starr, and Alexander D. Taylor as inventors, the applicationbeing incorporated herein by reference in its entirety.

This application relates to co-pending U.S. patent application Ser. No.08/881,925 attorney docket number SP-2078 US, filed on Jun. 25, 1997,entitled Broadly Distributed Termination For Buses Using SwitchedTerminator Logic and naming Jonathan E. Starr as inventor, theapplication being incorporated herein by reference in its entirety.

This application is a C-I-P of co-pending U.S. patent application Ser.No. 08/881,927 attorney docket number SP-2485 US, filed on Jun. 25,1997, entitled Method of Broadly Distributed Termination For Buses UsingSwitched Terminators and naming Jonathan E. Starr as inventor, theapplication being incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to buses and more particularly totermination of buses for use in information processing systems.

2. The Background Art

DESCRIPTION OF THE RELATED ART

In computer and information processing systems, various integratedcircuit chips communicate digitally with each other over a common bus.The signal frequency at which this communication occurs can limit theperformance of the overall system. Thus, the higher the communicationfrequency, the better. The maximum frequency at which a systemcommunicates is a function not only of the time that it takes for theelectromagnetic wavefronts to propagate on the bus from one chip toanother, but also of the time required for the signals to settle tolevels that can be recognized reliably at the receiving bus nodes asbeing high or low, referred to as the settling time.

The length of the settling time is a function of the amount ofreflection and ringing that occurs on the transmission line. The moreeffective the termination of a bus system, the smaller the effects ofreflection and ringing in the system and the shorter the overallsettling time of the signal.

SUMMARY OF THE INVENTION

An information handling system includes a plurality of transmissionlines coupled together at one end and having a characteristic impedance,a driver circuit coupled to one of the transmission lines, a pluralityof receiver circuits individually coupled to distinct ones of thetransmission lines for resolving the signals, and on-chip terminatorshaving a output impedance corresponding to the characteristic impedanceand that can be coupled or decoupled from a node by on-chip circuitry.In this embodiment, the terminators are separate and distinct fromdriver circuitry. However, when a node is in a receive configuration,its corresponding termination resistor is tied to the transmission lineat that node, and its corresponding driver circuit presents a highimpedance output to the node. A second embodiment provides drivercircuits having particular on-chip pull-up and pull-down resistances,one of the either the pull-up or pull-down resistances being used toterminate the bus for those nodes which are in a receive configuration.In this second embodiment, separate terminators are not required, due tothe driver operating as the terminator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a block diagram of an information handling system havingdriver, receiver, and termination circuits in accordance with thepresent invention.

FIG. 2 is a block diagram of an information handling system according tothe present invention.

FIGS. 3A, 3B, 3C, 3D, 3E, and 3F show schematic block diagrams of avariety of bit elements of the driver circuits in accordance with thepresent invention.

FIG. 4 is a block diagram of an alternate embodiment of the presentinvention.

FIG. 5A is a signal response curve for a system as depicted in FIG. 2showing a voltage overshoot condition. FIG. 5B is a signal responsecurve for a system such as is depicted in FIG. 4.

DETAILED DESCRIPTION OF ONE EMBODIMENT

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons having the benefit of thisdisclosure.

The use of the same reference symbols in different figures indicatessimilar or identical items.

FIG. 1 is a block diagram of an information handling system havingdriver, receiver, and termination circuits in accordance with thepresent invention.

Referring to FIG. 1, information handling system 100 includes aplurality of devices 102 which communicate with each other overtransmission lines 104.

Each component 102 includes a driver circuit 106, a receiver circuit 108and an optional terminator circuit 110. The output resistance of each ofdriver circuits 106 is controlled using impedance control circuitry asis known in the art. One such means of controlling the output resistanceof a driver is disclosed in U.S. patent application Ser. No. 08/881,939filed on Jun. 25, 1997, entitled Impedance Control Circuit and Sai V.Vishwanthaiah, Jonathan E. Starr, and Alexander D. Taylor as inventors,the application being incorporated herein by reference in its entirety.

Herein, a node is defined as an endpoint of a transmission line which isnot directly coupled to another transmission line. It may be coupled toone or more of a driver, receiver, or terminator circuit, or acombination of such circuits. For example, the system of FIG. 1 hasthree nodes.

A driver may comprise a pull-up circuit, a pull-down circuit, or both. Apull-up circuit, when enabled, couples the output node to the upperpower rail through a controlled output resistance; when disabled itpresents a high impedance to the output node. A pull-down circuit, whenenabled, couples the output node to the lower power rail through acontrolled output resistance; when disabled it presents a high impedanceto the output node. A driver transmits signals into the transmissionline by selectively enabling and disabling its pull-up or pull-downcircuits at the proper times.

In one embodiment, components 102 include terminators 110 which includean output resistance which is matched to the characteristic impedance ofthe transmission lines 104 and which is coupled to the transmission line104 whenever the corresponding driver circuit 106 is not drivingsignals. When a terminator 110 is enabled, the corresponding drivercircuit 106 provides a high impedance to the transmission line.

In this disclosure, when an output resistance is “matched” to thecharacteristic impedance of a transmission line, it is intended that theoutput resistance be within about ten percent of the characteristicimpedance of the transmission line.

In a second embodiment, terminator 110 is not present. Rather, theoutput resistance of the driver itself is used to terminate thetransmission line. That is, when not driving signals, the driverpresents a terminating resistance to the line which is approximatelyequal to the impedance of the transmission line.

Whether an information handling system has a separate terminator circuitor instead incorporates the termination resistances into the drivercircuitry, the effect on signals arriving at the end of the node is thesame.

FIG. 2 is a block diagram of an information handling system according tothe present invention.

In the system of FIG. 2, an embodiment is described wherein thetermination resistances are incorporated into the driver circuitry. Thatis, the driver output resistance is used to terminate the transmissionline. Those of ordinary skill in the art who are familiar with theinformation contained herein would be able to practice either the FIG. 2embodiment or an alternate embodiment which includes separateterminators 110 instead of incorporating the termination resistancesinto the drivers.

In the FIG. 2 embodiment, two distinct configurations are possible. Inthe first configuration, each non-driving node is connected to acorresponding driver whose pull-up circuit is enabled and acts toterminate the corresponding transmission line. In this firstconfiguration, R_(PU), the output resistance of the pull-up circuit, isapproximately equal to the transmission line impedance and R_(PD), theoutput resistance of the pull-down circuit, is approximately equal to avalue computed using a formula to be described below.

In the second configuration of FIG. 2, each non-driving node isconnected to a corresponding driver whose pull-down circuit is enabledand acts to terminate the corresponding transmission line. In thissecond configuration, R_(PU), the output resistance of the pull-upcircuit, is approximately equal to a value computed using a formula tobe described below and R_(PD), the output resistance of the pull-downcircuit, is approximately equal to the transmission line impedance.

In both configurations, the termination resistance is approximatelyequal to the transmission line impedance, and the termination resistanceis tied to the transmission line whenever the driver is not driving theline. When a node is in a driving configuration, the output resistanceof the driver at any given moment is determined by either R_(PU) whichis the output resistance of the driver's pull-up circuit when it isenabled, or R_(PD) which is the output resistance of the driver'spull-down circuit when it is enabled, or it is high impedance if neitherthe pull-up nor the pull-down circuit is enabled.

Referring to FIG. 2, each driver circuit 106 of information handlingsystem 100 includes a pull-up circuit circuit 202 and a pull-downcircuit circuit 204 coupled to transmission lines 104, transmissionlines 104 having a characteristic impedance of Z₀.

When a node is driving signals onto transmission line 104, the driver106 of the node either pulls the signal towards the upper power rail,pulls the signal towards the lower power rail or presents a highimpedance to the output node, whichever is appropriate to send thedesired signal. When a node is not driving, the pull-up portion (orpull-down portion, as appropriate) of the driver remains active, therebyproviding a terminating resistance which is matched to the impedance ofthe transmission line.

Because the impedance at each non-driving node is matched to theimpedance of the transmission line (due to the terminating resistance atthe driver circuit present at a non-driving node), signals arriving atthe non-driving node terminate without reflecting at that node. Thiscondition is highly desirable because the settling time of the signalvoltage level at the non-driving node is a critical parameter relatingto bus performance. The net effect of the absence of reflections fromthe stub ends is a reduced overall settling time when compared tosystems without termination at each non-driving node.

For the description which follows, it is assumed that any requiredtermination resistance is supplied by a driver output resistance whichis tied to the upper rail. Thus, R_(PU), the output resistance of adriver in the pull-up configuration, is approximately equal to thetransmission line impedance.

In the system of FIG. 2, the high signal voltage seen at a non-drivingnode is V_(dd). V_(dd) is the voltage of the upper power rail, andV_(ss) is the voltage of the lower power rail. The low signal voltageseen at a non-driving node is V_(dd)−V_(swing) where V_(swing) is givenby

V _(swing)=(V _(dd) −V _(ss))([Z ₀/(n−1)]/[{Z ₀/(n−1)}+R])  (R.1)

where R is the value of the pull-down resistance (shown as R_(PD) inFIG. 2) and where n is the number of nodes in the information handlingsystem 100. In the present invention, each node has a terminator ormeans for termination attached to it. The receivers are capable ofresolving the swing.

Solving for R gives

R=[Z ₀([(V _(dd) −V _(ss))/V _(swing)]−1)]/(n−1)  (R.2)

allowing for a particular pull-down resistance to be chosen for adesired V_(swing).

As an example, in the system of FIG. 2, assume that a voltage swing of(V_(dd)−V_(ss))/2 is desired. Thus, pull down driver circuit 204includes a pull-down resistance that is substantially equal to (i.e.within 10% of) $\begin{matrix}\begin{matrix}{R = {\lbrack {Z_{0}( {\lbrack {( {V_{dd} - V_{ss}} )/V_{swing}} \rbrack - 1} )} \rbrack/( {n - 1} )}} \\{= {\lbrack {Z_{0}( {\lbrack {{( {V_{dd} - V_{ss}} )/( {V_{dd} - V_{ss}} )}/2} \rbrack - 1} )} \rbrack/( {n - 1} )}} \\{= {{Z_{0}( {2 - 1} )}/( {n - 1} )}} \\{= {Z_{0}/( {n - 1} )}}\end{matrix} & ( {{from}\quad R{.2}} )\end{matrix}$

where n is the number of nodes in the information handling system 100.

Because the impedance at each non-driving node is matched to theimpedance of the transmission line (due to the terminating resistance atthe drivers located at those nodes), signals arriving at the non-drivingnode terminate without reflection. This condition is highly desirablebecause the settling time of the non-driving node is a criticalparameter relating to bus performance. The net effect of the absence ofreflections from the stub ends is a reduced overall settling time whencompared to systems without termination at each non-driving node.

In the FIG. 2 configuration, by having the pull-up resistors on chip,the pull-up resistance at the driving node is switched off when thedriver is pulling low. Accordingly, this system advantageously consumesless overall current and power than a system that has an off-chippull-up resistor that is always drawing current.

When the termination of a node takes place outside component 102, theterminator is separated from the non-driving node by some distance alonga transmission line and thus undesirable parasitics are introduced inthe connection to the termination resistor. Also, because of theseparation such a system can have reflections from the intersection ofthe stubs of the transmission line.

When the termination is within each component, the terminationresistance is placed right at the receiver, thereby reducing reflectionsand ringing. Thus, providing the termination within each component 102improves signal integrity when compared to terminating a node outsidecomponent 102.

Those of ordinary skill in the art will readily recognize that althoughthe embodiment of FIG. 2 described herein relates to a system having thetermination resistances tied to the upper rail, an alternate embodimentis contemplated which instead has the termination resistances tied tothe lower rail. In this alternate embodiment, the pull-down resistanceof each driver is approximately equal to the impedance of thetransmission line and the pull-up resistance is approximately equal tothe value of R given by relationship (R.2).

In an embodiment having separate terminators 110, the output resistanceof the terminator is approximately equal to the impedance of thetransmission line, and that resistance is connected to or disconnectedfrom the transmission line, depending on whether the particular node isreceiving or driving respectively. In this embodiment, the driver outputat a non-driving node is high impedance when the node is in a receivecondition.

Referring to FIGS. 3A, 3B, 3C and 3D, the pull-up and pull-down elementsof driver circuit 110 may be of a variety of configurations. Forexample, as shown in FIG. 3A, the driver element may be a PMOStransistor. Also for example, as shown in FIG. 3B, the driver elementmay be the parallel combination of a PMOS transistor and an NMOStransistor. With this parallel combination, it is the resistance of theparallel combination that would be equal to the desired bit elementresistance. Also for example, as shown in FIG. 3C, the driver elementmay be an NMOS transistor. Also for example, as shown in FIG. 3D, thedriver element may be the parallel combination of two NMOS transistors.In a preferred embodiment, the pull-up circuit element includes theparallel combination of the PMOS transistor and the NMOS transistor andthe pull-down element includes the parallel combination of two NMOStransistors. It will be appreciated that a driver circuit may have othercircuitry that contributes to the overall pull-up and pull-downresistance of the driver.

FIG. 4 is a block diagram of an alternate embodiment of the presentinvention.

Referring to FIG. 4, system 400 comprises drivers 402, 404 and 406individually coupled to resistance elements 408, 410, and 412 throughtransmission lines 414, 416, and 418, with each transmission line havinga characteristic impedance Z₀ and each resistance element having aresistance of R_(T). Drivers 402, 404, and 406 may include the exampleconfiguration of FIG. 2 where the pull-up circuit has an outputresistance which approximately equals the characteristic impedance ofthe line, and the pull-down circuit has an output resistance which isinversely proportional to the number of nodes, or may instead comprisethe reverse configuration described herein. Also, the system of FIG. 4may use separate terminators as previously described in relation to FIG.2.

To prevent signal reflections, the impedance looking into the resistivenetwork from one of the transmission lines must match the impedance ofthe transmission line. For example, signals propagating from driver 402on the transmission line 414 will not suffer reflections at theinterface between line 414 and resistor 408 if the net impedance seenlooking into resistor 408 at this interface is equal to the impedance oftransmission line 414.

The impedance seen looking into resistor 408, 410, or 412 from thecorresponding transmission line 414, 416, or 418 is

Z _(network) =R _(T)+[(R _(T) +Z ₀)/(n−1)]  (R.3)

Where n represents the number of nodes in the system.

In order to prevent reflections,

Z_(network)=Z₀  (R.4)

In order to match impedances, relationship (R.4) is substituted intorelationship (R.3), resulting in

R _(T) =Z ₀×[(n−2)/n]  (R.5)

Thus, in the three-node system of FIG. 4, each resistor in the resistornetwork has a value of $\begin{matrix}{R_{T} = {Z_{0} \times \lbrack {( {n - 2} )/n} \rbrack}} \\{= {Z_{0} \times \lbrack {( {3 - 2} )/3} \rbrack}} \\{= {{Z_{0}/3}\quad \text{ohms}}}\end{matrix}$

Thus, a signal propagating away from any of drivers 402, 404, or 406will see, looking into the resistor network, a net impedance of

Z _(network) =R _(T)+0.5×(R _(T) +Z ₀)=Z ₀/3+2Z ₀/3=Z ₀

In the system of FIG. 4, the voltage swing at a non-driving node isgiven by

V _(swing)=(V _(dd) −V _(ss))(Z ₀/[(R+Z ₀)(n−1)])  (R.6)

where R is the output resistance of the driver pull-down circuit if theterminating resistances at non-driving nodes are coupled to the upperpower rail; otherwise, R is the output resistance of the driver pull-upcircuit if the terminating resistances at non-driving nodes are coupledto the lower power rail.

Using the previously described example configuration of FIG. 2 with theresistor network of FIG. 4, the value of the pull-down resistance isdetermined by solving relationship (R.6) for R, giving

R=Z ₀([(V _(dd) −V _(ss))−V _(swing)(n−1)]/[V _(swing)(n−1)])  (R.7)

In systems such as the system of FIG. 2 (lacking a resistive networksuch as that seen in FIG. 4), a signal propagating from a driver towardsthe common point 214 will see an impedance at the common point which isnot matched to the impedance of the transmission line. Therefore, inthose systems, a reflection would occur at common connection point 214.The magnitude of that reflection increases as the impedance mismatchincreases. In systems employing the embodiment of FIG. 4, the impedanceseen at the connection point between the resistor and the transmissionline by a signal propagating from a node to common connection point 420is equal to the impedance of the transmission line, so no reflectionoccurs. Since no reflection occurs, a second signal may be sent soonerthan otherwise possible, thus increasing signaling frequency of thesystem.

A significant benefit resulting from using the FIG. 4 embodiment of thepresent invention is that signals may be pipelined. Pipelining is when asignal is launched on a signal conductor prior to the most recentlylaunched signal arriving at a receiving end. In some systems, includingthe system of FIG. 2, in order to transmit a signal, it may be necessaryto wait until the magnitudes of the reflections on the driver stubresulting from a previously launched signal reflecting from the junctionhave diminished enough to not interfere with subsequently launchedsignals. Using the system of FIG. 4, there are no reflections. Thereforea signal may be launched by a driver prior to the time a previouslylaunched signal has arrived at the receiving ends.

A second benefit of the FIG. 4 embodiment is that the voltage present atany given time on the lines will not exceed the driving voltage, V_(dd).

FIG. 5A is a signal response curve for a system as depicted in FIG. 2showing a voltage overshoot condition. FIG. 5B is a signal responsecurve for a system such as is depicted in FIG. 4.

Both signal response curves show the voltage at the driving andreceiving ends where V_(dd)=1.5V, V_(ss)=0V, the transmission lineimpedance is 50 ohms, the pull-down resistance is 25 ohms, and thepull-up resistance is 50 ohms.

Referring to FIG. 5A, from time t₀ to just prior to time t₁, the driveris pulling low. While the driver, assumed to be chip 1 of FIG. 2, ispulling low, there is a 25 ohm pull-down resistance in series with aparallel combination of the 50 ohm pull-up resistance of chip 2 of FIG.2, and a 50 ohm pull-up resistance of chip 3 of FIG. 2. Therefore, thecurrent is V_(dd)/R=1.5V/50 ohms=30 ma. The initial voltage at alldriver and receiver ends is V_(dd)/2.

At approximately time t₁, the driver no longer pulls low, and is in thetransition stage to pulling high. At this time, the 30 ma current flowstops, giving a ΔI of 30 ma. Since Z₀=50 ohms, there is a ΔV of(Z₀)(ΔI)=1.5V. Now, the total voltage at the driving end is the sum ofthe initial voltage plus ΔV=0.75V+1.5V=2.25V. However, when the 50 ohmpull-up element turns on, there is a 1:1 voltage divider between thepull-up resistance and the line, resulting in a divider between 1.5V and2.25V, resulting in a total line voltage of 1.875V. Thus, the totalvoltage on the line exceeds V_(dd).

Between time t₁ and just after time t₃, the voltage on the lineovershoots V_(dd), and could cause damage to driving elements that arenot protected against such damage.

At approximately time t₂, the driven signal arrives at junction 214. Dueto the impedance mismatch there, a negative reflection of the signalpropagates back towards the driver. It is the reflected signal which,upon arriving back at the driver, brings the voltage at the driver enddown from the overshoot condition seen at approximately times t₁ to t₃to V_(DD) at approximately time t₄.

Although the example signal response curve for FIG. 5A pertains to asystem having a total of three nodes, the curve of FIG. 5A alsogenerally applies to systems having more nodes than three. The primarydifference in the curve for many nodes would be the actual overshootvoltage.

FIG. 5B utilizes the configuration described with respect to FIG. 4. Asshown, this new configuration does not have an overshoot condition whichwould otherwise damage components that are not sturdy enough to toleratea voltage overshoot without becoming damaged.

Referring to FIG. 5B, from time t₀ to just prior to time t₁, the driveris pulling low. It can be shown that the driver current is given by

I _(driver) =V _(dd)/(R+Z ₀)  (R.8)

The voltage out of the driver at time t₀ is simply the currentmultiplied by the driver resistance R, giving

V _(driver) =[V _(dd)/(R+Z ₀)]R  (R.9)

Thus, when the driver stops pulling low, the change in current is givenby relationship (R.8). The change in voltage in the system is$\begin{matrix}\begin{matrix}{{\Delta V} = {Z_{0}({\Delta I})}} \\{= {Z_{0}\lbrack {V_{dd}/( {R + Z_{0}} )} \rbrack}}\end{matrix} & ( {R{.10}} )\end{matrix}$

The voltage after the driver stops pulling low is merely the sum of theoriginal voltage and the change in voltage (R.9 and R.10), giving

=[V _(dd)/(R+Z ₀)]R+Z ₀ [V _(dd)/(R+Z ₀)]

=(R+Z ₀)[V _(dd)/(R+Z ₀)]

=V_(dd)

showing that there is no overshoot with the system of FIG. 4.

Although the example signal response curve for FIG. 5B pertains to asystem having a total of three nodes, the curve of FIG. 5B alsogenerally applies to systems having more nodes than three.

Those of ordinary skill in the art will readily recognize that althoughthe examples given herein have employed three nodes, systems having morenodes than three are contemplated.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art that manymore modifications than mentioned above are possible without departingfrom the inventive concepts herein. The invention, therefore, is not tobe restricted except in the spirit of the appended claims.

What is claimed is:
 1. An information handling system comprising: atleast a first, a second, and a third transmission line each having acharacteristic impedance, each of said transmission lines having a firstend and a second end, each of said first ends being commonly connectedto each other; a driver circuit connected to said second end of saidfirst said transmission line; a first receiver circuit connected to saidsecond end of said second transmission line; a second receiver circuitconnected to said second end of said third transmission line; aplurality of on-chip terminators each terminating said second end of oneof said second and third transmission lines, each of said on-chipterminators having an output resistance matched to the characteristicimpedance of one of said second and third the transmission lines; saiddriver circuit further comprises a pull-up circuit and said terminatorsare connected to a lower power rail; and said pull-up circuit has anoutput resistance corresponding to the number of second ends of saidtransmission lines.
 2. The information handling system of claim 1,wherein said output resistance of said pull-up circuit is within tenpercent of [Z₀([V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ is acharacteristic impedance of said transmission lines, n is the number ofsecond ends of said transmission lines, V_(dd) is a voltage at an upperrail, V_(ss) is a voltage at said lower rail, and V_(swing) is a voltageswing at said second end of one of said second and third transmissionlines.
 3. The information handling system of claim 1 wherein said drivercircuit comprises a pull-down circuit and said terminators are coupledto an upper power rail.
 4. The information handling system of claim 3wherein said pull-down circuit has an output resistance corresponding tothe number of second ends.
 5. The information handling system of claim4, wherein said output resistance of said pull-down circuit is withinten percent of [Z₀([(V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ isa characteristic impedance of said transmission lines, n is the numberof second ends of said transmission lines, V_(dd) is a voltage at anupper rail, V_(ss) is a voltage at said lower rail, and V_(swing) is avoltage swing at said second end of one of said second and thirdtransmission lines.
 6. An information handling system comprising: atleast three transmission lines each having a characteristic impedance,each of said transmission lines having a first end, each of said firstends being commonly connected to each other first end, and a second end;a plurality of drivers equaling the number of transmission lines in saidplurality of transmission lines, each driver coupled to a second end ofa different one of said transmission lines, each driver comprising apull-down circuit having a pull-down resistance matched to thecharacteristic impedance of one of said lines, and a pull-up circuit forpulling up a signal on one of said lines, the pull-up circuit having apull-up resistance corresponding to the number of second ends; and atleast one receiver circuit coupled to a second end of one of saidtransmission lines.
 7. The information handling system of claim 6,wherein the output resistance of said pull-up circuit is within tenpercent of [Z₀([(V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ is acharacteristic impedance of said transmission lines, n is the number ofsecond ends of said transmission lines, V_(dd) is a voltage at an upperrail, V_(ss) is a voltage at said lower rail, and V_(swing) is a voltageswing at said second end of one of said second and third transmissionlines.
 8. The information handling system of claim 6 wherein the pull-upcircuit comprises a PMOS transistor.
 9. The information handling systemof claim 6 wherein the pull-down circuit comprises a PMOS transistor.10. The information handling system of claim 6 wherein the pull-upcircuit comprises an NMOS transistor.
 11. The information handlingsystem of claim 6 wherein the pull-down circuit comprises an NMOStransistor.
 12. The information handling system of claim 6 wherein thepull-down circuit comprises the parallel combination of a PMOStransistor and an NMOS transistor.
 13. The information handling systemof claim 6 wherein the pull-up circuit comprises the parallelcombination of a PMOS transistor and an NMOS transistor.
 14. Theinformation handling system of claim 6, wherein the pull-down circuitcomprises the parallel combination of two NMOS transistors wherein afirst one of said two NMOS transistors is diode connected.
 15. Theinformation handling system of claim 6, wherein the pull-up circuitcomprises the parallel combination of two PMOS transistors wherein afirst one of said two PMOS transistors is diode connected.
 16. Theinformation handling system of claim 6 wherein the pull-up circuitwithin a given driver is enabled and therefore acts as a terminatorwhenever the given driver circuit is not being used to drive thetransmission line.
 17. The information handling system of claim 6further comprising a plurality of on-chip terminators, each of saidon-chip terminators being individually coupled between a lower powerrail and different ones of said transmission lines when the node towhich a given terminator is connected is in a nondriving configurationand the corresponding driver is not enabled to act as a terminator, saidterminators having an output resistance matched to the characteristicimpedance of the transmission line.
 18. An information handling systemcomprising: at least three transmission lines each having acharacteristic impedance, each of said transmission lines having a firstend, each of said first ends being commonly connected to each otherfirst end, and a second end; a number of drivers equaling the number oftransmission lines in said plurality of transmission lines, each drivercoupled to a second end of a different one of said transmission lines,each driver comprising a pull-up circuit having a pull-up resistancematched to the characteristic impedance of one of said lines, and apull-down circuit for pulling down a signal on one of said lines, thepull-down circuit having a pull-down resistance corresponding to thenumber of second ends; and at least one receiver circuit coupled to asecond end of one of said transmission lines.
 19. The informationhandling system of claim 18, wherein said output resistance of saidpull-down circuit is within ten percent of[Z₀([(V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ is acharacteristic impedance of said transmission lines, n is the number ofsecond ends of said transmission lines, V_(dd) is a voltage at an upperrail, V_(ss) is a voltage at said lower rail, and V_(swing) is a voltageswing at said second end of one of said second and third transmissionlines.
 20. The information handling system of claim 18 wherein thepull-down circuit comprises a PMOS transistor.
 21. The informationhandling system of claim 18 wherein the pull-up circuit comprises a PMOStransistor.
 22. The information handling system of claim 18 wherein thepull-down circuit comprises a NMOS transistor.
 23. The informationhandling system of claim 18 wherein the pull-up circuit comprises a NMOStransistor.
 24. The information handling system of claim 18 wherein thepull-up circuit comprises the parallel combination of a PMOS transistorand an NMOS transistor.
 25. The information handling system of claim 18wherein the pull-down circuit comprises the parallel combination of aPMOS transistor and an NMOS transistor.
 26. The information handlingsystem of claim 18, wherein the pull-down circuit comprises the parallelcombination of two NMOS transistors wherein as first one of said twoNMOS transistors is diode connected.
 27. The information handling systemof claim 18, wherein the pull-up circuit comprises the parallelcombination of two PMOS transistors wherein a first one of said two PMOStransistors is diode connected.
 28. The information handling system ofclaim 18 wherein the pull-up circuit within a given driver is enabledand therefore acts as a terminator whenever the given driver circuit isnot being used to drive the transmission line.
 29. The informationhandling system of claim 18 further comprising a plurality of on-chipterminators, each of said on-chip terminators being individually coupledbetween an upper power rail and different ones of said transmissionlines when the node to which a given terminator is connected is in anondriving configuration and a driver present at that node is notenabled to act as a terminator, said terminators having an outputresistance matched to the characteristic impedance of the transmissionline.
 30. An information handling system comprising: at least threetransmission lines each having a characteristic impedance, each of saidtransmission lines having a first end, each of said first ends beingcommonly connected to each other first end, and a second end; aplurality of drivers, each driver individually coupled to a second endof one of said transmission lines, each driver comprising a pull-downcircuit having a pull-down resistance matched to the characteristicimpedance of one of said transmission lines, and a pull-up circuit forpulling up a signal on the one of said transmission lines to which thedriver is connected, the pull-up circuit having a pull-up resistancecorresponding to the number of second ends; at least one receivercircuit coupled to a second end of one of said transmission lines; and aplurality of on-chip terminators, each of said on-chip terminators beingindividually coupled between a lower power rail and different ones ofsaid transmission lines when the node to which a given terminator is ina nondriving configuration, said terminators having an output resistancematched to the characteristic impedance of the transmission line. 31.The information handling system of claim 30, wherein said outputresistance of said pull-down circuit is within ten percent of[Z₀([(V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ is acharacteristic impedance of said transmission lines, n is the number ofsecond ends of said transmission lines, V_(dd) is a voltage at an upperrail, V_(ss) is a voltage at said lower rail, and V_(swing) is a voltageswing at said second end of one of said second and third transmissionlines.
 32. An information handling system comprising: at least threetransmission lines each having a characteristic impedance, each of saidtransmission lines having a first end, each of said first ends beingcommonly connected to each other first end, and a second end; aplurality of drivers, each driver coupled to a second end of differentones of said transmission lines, each driver comprising a pull-upcircuit having a pull-up resistance matched to the characteristicimpedance of one of said transmission lines, and a pull-down circuit forpulling down a signal on one of said transmission lines, the pull-downcircuit having a pull-down resistance corresponding to the number ofsecond ends; and at least one receiver circuit coupled to a second endof one of said transmission lines; and a plurality of on-chipterminators, each of said on-chip terminators being coupled between anupper power rail and different ones of said transmission lines when thenode to which a given terminator is in a nondriving configuration, saidterminators having an output resistance matched to the characteristicimpedance of the transmission line, wherein at least one on-chipterminator is coupled to those second ends not having a driver coupledthereto.
 33. The information handling system of claim 32, wherein saidoutput resistance of said pull-down circuit is within ten percent of[Z₀([(V_(dd)−V_(ss))/V_(swing)]−1)]/(n−1), wherein Z₀ is acharacteristic impedance of said transmission lines, n is the number ofsecond ends of said transmission lines, V_(dd) is a voltage at an upperrail, V_(ss) is a voltage at said lower rail, and V_(swing) is a voltageswing at said second end of one of said second and third transmissionlines.